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This is slightly off topic, but I was told by a botanist that digitalis comes true from seed and that they don't mix with other species.
I found no description and even no author attached to the name. It is not listed in the Plant list nor in scientific papers about phylogeny of foxgloves and turkish foxgloves. So, I 'm now quite sure that there is no Digitalis stewartii (and it's even not a synonym since there is no description) and that the true name of this foxglove should be Digitalis ferruginea. Indeed, I looked again at Eker's paper and there is a map of turkish foxgloves : at Dilek Peninsula, there is only D. ferruginea ssp. ferruginea and D. cariensis.... D. f. schishkinii is distributed in north-east Anatolia. So, yours D. stewartii should be D. ferruginea ferruginea. Other might be D. f. schischkinii or even hybrid with D. laevigata ?
Yvain mentioned the Phytotaxa article a long ago, but apparently no one payed attention, so here is the link. http://biotaxa.org/Phytotaxa/article/view/phytotaxa.244.3.3
I have the PDF of this article from İsmail Eker and Researchgate (ask me by PM if interested). The phylogenies and the plates of the digitalis showing the variability of Digitalis ferruginea can be looked here : https://www.researchgate.net/publication/291140350_Phylogeny_of_Anatolian_Turkey_species_in_the_Digitalis_sect_Globiflorae_Plantaginaceae
This digitalis is from seed ex seed labelled D. dubia. I don't think that is correct after looking up pics on google, so what do you reckon it is please?